This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. The word or words serving to define another word or expression, as in a dictionary entry.

n. The word or phrase that defines the definiendum in a definition.

"More generally, it’s the expression to be defined, as opposed to the definiens, which is the phrase doing the defining."

define    definition    definiendum